high-fidelity vs hi-fi

high-fidelity is not a noun while hi-fi is a noun.

Word	Noun	Adjective	Verb	Adverb
high-fidelity	No	Yes	No	No
hi-fi	Yes	Yes	No	No

As adjectives, high-fidelity and hi-fi are synonyms defined as:

high-fidelity and hi-fi: characterized by minimal distortion in sound reproduction

high-fidelity (noun)	hi-fi (noun)
	the reproduction of sound with little or no distortion
	equipment for the reproduction of sound with high fidelity

high-fidelity (adjective)	hi-fi (adjective)
characterized by minimal distortion in sound reproduction	characterized by minimal distortion in sound reproduction